METAZOA PORIFERA. 65 



blastula become differentiated and turn inward or become 

 invaginated, and the embryo possesses two layers (the 

 outer layer or ectoderm and the inner layer or endoderm), 

 a gastral cavity or archenteron, with an opening, the blas- 

 topore. As the process of digestion was supposed to go 

 on in the gastral cavity, the embryo at this stage was 

 called the gastrula. 



It will be noticed, that, according to this theory, the 

 invaginated gastrula represents a primitive stage in the 

 development, arising directly from the blastula ; also that 

 the archenteron and blastopore are primitive and not 

 secondarily acquired characters. Furthermore, the proc- 

 ess of invagination, by which these conditions have been 

 brought about] must of course, according to this theory, 

 be a primitive process. Haeckel maintains that the proc- 

 ess of delamination or the cross division of cells, to be 

 spoken of hereafter, is a modification of invagination, but 

 does not show how the one is derived from the other. 



Extended observations on sponges and Coelentera 

 proved that the occurrence of the invaginated gastrula 

 was exceptional instead of normal, as would be expected 

 in these, the simplest and most generalized groups. In 

 Ascetta primordialis, one of the simplest calcareous 

 sponges, and in the silicious and horny sponges, in the 

 Hydrozoa and Anthozoa, the stage following the blastula 

 is not, as a rule, an invaginated gastrula but something 

 quite different. It is a solid, mouthless embryo, consist- 

 ing of one layer of cells on the periphery and a mass of 

 cells in the interior. 



The parenchymella theory of Metschnikoff throws light 

 on the origin of this stage of development. The blastula, 

 according to this author, is converted into the solid em- 

 bryo or parenchymella 1 by the process of immigration of 

 cells from the surface (such as was seen in Proterospongia 



1 We use parenchymella (Metschnikoff) instead of planula (Lan- 

 kester) because the theory of Metschnikoff is given the preference 

 to that of Lankester. (See McMurrich, Biol. Lect. Mar. Biol. Lab.. 

 Wood's Hole, 1891.) 



